Remove v1alpha1 protos and related code. (#868)

If needed, this code can be added to registry-experimental.

Author: timburks

cmd/registry/breaking-change-detector/breaking_change_detector.go

@@ -28,7 +28,6 @@ func Command() *cobra.Command {
 	cmd.AddCommand(complexityCommand())
 	cmd.AddCommand(lintCommand())
 	cmd.AddCommand(lintStatsCommand())
-	cmd.AddCommand(referencesCommand())
 	cmd.AddCommand(scoreCommand())
 	cmd.AddCommand(scoreCardCommand())
 	cmd.AddCommand(vocabularyCommand())